If a printhead having a plurality of nozzles in an inkjet printer includes even one discharge failure nozzle, a white stripe appears on a printed product, and the printed product cannot be used formally. When even one discharge failure nozzle exists in the printhead and the discharge failure occurs due to a cause which cannot be solved even by a recovery process, there has conventionally been no method of solving this printing failure except that the printhead having the discharge failure nozzle is not used. More specifically, when an unrecoverable discharge failure nozzle is detected during the manufacture of a printhead, the printhead having the discharge failure nozzle must be discarded. If a discharge failure nozzle unrecoverable by the recovery process is generated in the printhead after the printhead is passed to the user, the user must exchange the printhead.
Not only a discharge failure nozzle, but also a nozzle which cannot correctly print due to a discharge direction greatly deviated from a normal direction, and a nozzle which influences printing because the size of a discharged ink droplet is greatly different from a normal one are not suitable for normal printing. These nozzles are treated as abnormal nozzles, similar to the discharge failure nozzle. A printhead having such abnormal nozzle is regarded as a defective printhead.
This situation, i.e., generation of a discharge failure nozzle (to be also referred to as an abnormal nozzle hereinafter) in the printhead imposes an economic burden on both the printer manufacturer and user.
Recent printheads are equipped with a large number of printing nozzles. Some printheads have 512 nozzles per color, and when many nozzles are arranged for six colors, the printhead has a total of 3,072 nozzles. As the number of nozzles increases, the possibility at which discharge failure nozzles occur increases. Demands have arisen for a measure against a discharge failure nozzle so as to reduce the economic burden on both the printer manufacturer and user.
In order to avoid this situation, several printer manufacturers have recently proposed a technique associated with so-called discharge failure complement of complementing printing data of a discharge failure nozzle in the printhead. These proposals are similar to each other, and a representative example of the reference is Japanese Patent Publication Laid Open No. 6-226982. A feature of this technique is to print, when a discharge failure nozzle exists in the printhead, printing data at the position of the discharge failure nozzle by a normal nozzle.
For example, in multi-pass printing according to the complement method by a normal nozzle at the printing data position of a discharge failure nozzle, printing is done by one scanning in the main scanning direction, and then the sheet is fed in the sub-scanning direction. At this time, the sheet is not fed by the length of the printhead in the sub-scanning direction. In general, the sheet is fed by only a length obtained by dividing the length of the printhead by the multi-pass count. More specifically, when the printhead has 512 nozzles and performs printing which is completed by four passes, the sheet feed amount after one scanning in the main scanning direction is almost equal to a printhead length of 512÷4=128 nozzles. At this time, the same raster on the sheet surface is always printed by different nozzles of the printhead in respective passes. In the above example of 4-pass printing using 512 nozzles, a raster printed by the first nozzle counted from the upper end of the printhead in the first pass shifts by 128 nozzles in the second pass, and is identical to a raster printed by the 129th nozzle counted from the upper end of the printhead. From this principle, when the first nozzle counted from the upper end of the printhead is a discharge failure nozzle, data to be printed by the first nozzle is printed by the 129th nozzle counted from the upper end of the printhead in the second pass of 4-pass printing. In this manner, printing can be achieved by complementing the discharge failure of the first nozzle.
Also in single-pass printing, a discharge failure can be complemented in principle by setting a discharge failure complement printing pass in addition to a normal printing pass. Also in the above example, when the printhead has 512 nozzles and the first nozzle counted from the upper end of the printhead is a discharge failure nozzle, single-pass printing is normally executed in the first pass. The sheet is then fed by a printhead length of 128 nozzles, and the 129th nozzle counted from the upper end of the printhead prints data of the first nozzle. At this time, no printing is done by another nozzle, thereby implementing complement of a discharge failure.
There is also known an arrangement in which nozzles other than a discharge failure nozzle print in main scanning in the forward direction, then the sheet is slightly fed, and other nozzles print in an area not printed due to a discharge failure in scanning the carriage in the backward direction (see, e.g., Japanese Patent Publication Laid Open No. 8-25700).
To complement a discharge failure by the conventional method, the carriage must be scanned at least twice in the main scanning direction.
As another discharge failure complement method, for example, Japanese Patent Publication Laid Open No. 2002-19101 discloses a method of performing complement in the same scanning using a nozzle of another color, and a method of increasing the printing duty of a nozzle adjacent to a discharge failure nozzle and complementing a part which is not printed owing to a discharge failure.
Japanese Patent Publication Laid Open No. 06-079956 discloses an arrangement in which a printing block and complementary block are prepared, and when an abnormal nozzle is generated in the printing block, it is complemented by the nozzle of the complementary block.
Japanese Patent Publication Laid Open No. 09-174824 discloses an arrangement in which printing is performed using part of a nozzle array except its end, and when a discharge failure occurs at the end of the part used, discharge failure complement is done using an unused part.
However, the conventional discharge failure complement technique poses the following problems.
Multi-pass printing will be considered. One of printing methods often adopted in current printers is margin-less printing. In this printing mode, for A4 size, printing is done on the entire sheet surface of this size. Generally in this printing at portions corresponding to the upper and lower margins (margins in the sub-scanning direction) of a paper sheet, the sheet feed amount changes even by using the same multi-pass. In the above example of 4-pass printing using 512 nozzles, the sheet feed amount is almost equal to a printhead length of 128 nozzles, as described above. At portions corresponding to the upper and lower margins of a paper sheet, printing is done using not all the 512 nozzles, but only some nozzles, e.g., 128 nozzles. At this time, the sheet feed amount is 128÷4=32 nozzles. With this setting, a raster printed by the first nozzle counted from the upper end of the printhead shifts by 32 nozzles in the second pass, and is identical to a raster printed by the 33rd nozzle counted from the upper end of the printhead. According to this principle, the position of a complementable nozzle dynamically changes on the same printing sheet surface, unlike the case in which, when the first nozzle counted from the upper end of the printhead is a discharge failure nozzle, similar to the above-described example, it is unconditionally decided that data of the discharge failure nozzle can be complemented by the 129th nozzle counted from the upper end of the printhead. It is a heavy burden on the system to process in real time to a certain degree the dynamic relationship between a discharge failure nozzle and a complementary nozzle. A discharge failure cannot be complemented in practice if discharge failure nozzles exist in different nozzle arrays of the same printhead.
Discharge failure complement in the conventional single-pass printing described above requires redundant scanning in the main scanning direction for only the complement process, which actually decreases the printing speed.
In the method of complementing a discharge failure by using a nozzle adjacent to the discharge failure nozzle, the use frequency of the nozzle adjacent to the discharge failure nozzle excessively rises and greatly degrades due to the difference in use frequency from another nozzle not used for complement. This may lead to a short service life of the printhead, and this problem is desirably solved in an application to actual products.
As a method of eliminating redundant scanning in the main scanning direction for only the complement process, there is proposed the following discharge failure complement method. That is, discharge failure complement is completed not by multiple passes but by only one scanning in the main scanning direction. More specifically, when a discharge failure nozzle exists in the printhead, printing data assigned to this nozzle is distributed to a normal printing nozzle of the same nozzle array that exists near the discharge failure nozzle. This method can eliminate the need for a complicated data process over multiple passes even in discharge failure complement. No printing pass for only discharge failure complement exists, and a high-speed process can be easily achieved at a relatively low cost.
However, the conventional technique of completing discharge failure complement by only one scanning in the main scanning direction suffers the following problems.
That is, the method of distributing printing data assigned to a discharge failure nozzle to a normal printing nozzle of the same nozzle array that exists near the discharge failure nozzle generates nozzle positions at which discharge failure complement is physically impossible. These nozzle positions correspond to upper and lower nozzles.
For example, assume that a discharge failure occurs at the first or 512th nozzle at the upper or lower end of the head when the printhead has 512 nozzles per nozzle array. If the first nozzle is a discharge failure nozzle, it can be complemented by only a nozzle such as the second or third nozzle in a direction in which the nozzle number increases. This is because the printhead does not have any 0th or −1st nozzle. If the 512th nozzle is a discharge failure nozzle, it can be complemented by only a nozzle such as the 511th or 510th nozzle in a direction in which the nozzle number decreases. This is because the printhead does not have any 513th or 514th nozzle.
In this case, the nozzle position subjected to discharge failure complement shifts to the upper or lower nozzle of a discharge failure nozzle (which of the second and third nozzles corresponds to an upper or lower nozzle for the first nozzle, or which of the 511th and 510th nozzles corresponds to an upper or lower nozzle for the 512th nozzle depends on the design of the head structure and assignment of the nozzle number). This leads to degradation of the quality of a printed image. In discharge failure complement, the highest image quality is obtained only when upper and lower nozzles of a discharge failure nozzle can be uniformly used.
As described above, although the discharge failure complement method has conventionally been proposed, a further improvement is required in the implementation. Especially, an effective discharge failure complement technique of suppressing a decrease in printing speed by a simple method must be established.